Extending the Read Range of Passive RFID Tags

ABSTRACT

An embodiment of the present invention improves the efficiency of radio frequency identification (RFID) systems and helps to extend the effective read range for certain configurations of closely spaced RFID tags. Specifically, an embodiment helps to minimize energy losses that result when there is excess energy from the excitation source. Various embodiments are directed toward using only as much of the excitation energy as necessary to power the RFID circuitry. An embodiment may have the benefit of making more of the excitation energy available to power other RFID tags nearby—thereby improving system performance and read range.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 12/392,736 titled “Extending the Read Range of Passive RFID Tags”filed Feb. 29, 2009, which claims priority to U.S. ProvisionalApplication No. 61/031,270 titled “Dynamic Power Absorption of a LoopAntenna for Passive RFID Tags” filed Feb. 25, 2008, and to U.S.Provisional Application No. 61/046,671 titled “Dynamic Power Regulationof a Loop Antenna for Passive RFID Tags Using a Varactor” filed Apr. 21,2008, and which is a continuation in part (CIP) of U.S. patentapplication Ser. No. 12/351,774 titled “Enhancing the Efficiency ofEnergy Transfer to/from Passive ID Circuits Using Ferrite Cores” filedJan. 9, 2009, which are incorporated herein by reference.

BACKGROUND

The present invention relates to passive radio frequency identification(RFID) tags, and in particular, to improved power efficiencies inpassive RFID tags by minimizing excess power consumption.

Unless otherwise indicated herein, the approaches described in thissection are not prior art to the claims in this application and are notadmitted to be prior art by inclusion in this section.

Typical RFID tags consist of an RFID chip mounted on an inlay with anantenna. The antenna is tuned to maximize sensitivity (and hence readrange) at a specified operating frequency. In normal operation, anexcitation source (e.g., RFID reader) generates a carrier frequency thatenergizes the RFID tag and bi-directional data is superposed on thiscarrier. One or two volts is usually sufficient to power the RFID chip.

When a tag is very close to the excitation source, the voltage generatedby its antenna can potentially exceed the rating of the RFID circuit. Ashunt regulator is the most common way to protect the circuit againstthis potential source of damage. This shunt regulator protects the RFIDcircuit by dumping the excess energy as heat. For most RFIDapplications, this energy loss does not meaningfully detract from systemperformance.

Some RFID applications, however, require reading tags that are closelyspaced. Unfortunately, closely spaced RFID tags tend to couple, shiftingtheir resonance frequency from that of the carrier to some unknownfrequency. This shift causes a breakdown in the transfer of energy anddata between reader and tags. The more closely coupled the tags, thegreater the shift in resonance, and the greater the degradation insystem performance.

One solution that has been developed—with limited results—is to not tunethe RFID tag to minimize the coupling between adjacent tags. See, forexample, products from Magellan Technology Pty Ltd, Sydney, Australia.However, this solution has poor spatial discrimination. Attempts toimprove spatial discrimination may themselves introduce further issues,such as collapsing the field and severely limiting the read range. Tocounter the limited read range, various range extension solutions may beimplemented. Unfortunately, when these range extension solutions arecombined with closely spaced “off-the-shelf” RFID tags, the shuntregulator exhibits a self-limiting behavior. Specifically, the shuntregulator in the tags nearest the excitation source (the reader) clampthe voltage at the output of the coil antenna in the tag. Since the tagsare closely spaced, the voltage available to nearby tags is limited tothis clamped voltage. Any losses (and there are always losses) furtherreduce the available voltage to where the tags do not have sufficientpower to operate. As noted above, this shunt regulator acts to protectthe RFID circuit (a good thing) but—in the case of closely spacedtags—the shunt regulator reduces the read range of the system.

Thus, there is a need for circuits that protect the sensitive circuitryin RFID tags without burning up excess energy as heat.

SUMMARY

An embodiment of the present invention improves the efficiency of radiofrequency identification (RFID) systems and helps to extend theeffective read range for certain configurations of closely spaced RFIDtags. Specifically, an embodiment helps to minimize energy losses thatresult when there is excess energy from the excitation source. Thisexcess energy has the potential to damage the circuitry in RFID tags. Ashunt regulator is often used to protect the RFID circuitry by clampingthe voltage, but has the undesirable effect of converting this excessexcitation energy into heat. Various embodiments are directed towardusing only as much of the excitation energy as necessary to power theRFID circuitry. These embodiments include circuitry for a constantcurrent power supply that can be implemented either with discretecomponents or built into a new ASIC design; a network that minimizes theamount of energy that is converted to heat by the shunt regulator builtinto many RFID circuits; and/or a sleep mode for reducing the energyconsumed by an RFID tag after it has been read. An embodiment may havethe benefit of making more of the excitation energy available to powerother RFID tags nearby—thereby improving system performance and readrange.

According to an embodiment, a method of performing radio frequencyidentification (RFID) includes (a) generating, by an RFID reader, anexcitation signal that includes a first read command. The method furtherincludes (b) generating, by a first plurality of RFID tags, a firstplurality of responses as part of the excitation signal. The methodfurther includes (c) decoding, by the RFID reader, the first pluralityof responses from the excitation signal. The method further includes (d)transmitting, by the RFID reader, a sleep command to each of the firstplurality of RFID tags using the excitation signal. The method furtherincludes (e) decoupling, by the first plurality of RFID tags, from anexcitation field generated by the excitation signal in response toreceiving the sleep command. The method further includes repeating (a),(b), (c), (d) and (e) for a second plurality of RFID tags.

According to an embodiment, an apparatus includes a circuit forperforming radio frequency identification (RFID). The circuit includesan antenna, and RFID electronics coupled to the antenna that activatesin response to receiving an excitation signal, that generates a responseas part of the excitation signal, that receives a sleep command as partof the excitation signal, and that decouples from an excitation fieldgenerated by the excitation signal in response to the sleep command.

According to an embodiment, an apparatus includes a circuit forperforming radio frequency identification (RFID). The circuit includesan antenna that receives energy from an excitation source, RFIDelectronics, and a linear power supply coupled between the antenna andthe RFID electronics. The linear power supply extracts from the energy aconstant current to operate the RFID electronics, wherein the constantcurrent is sufficient to power the RFID electronics and manages anyexcess excitation energy such that energy consumption is reduced. Theenergy consumption is reduced such that it is less than that of anon-constant current design exposed to a similar excitation field (i.e.,the energy consumption is less than an unmodified energy consumption,wherein the unmodified energy consumption corresponds to an unmodifiedapparatus having the antenna and the RFID electronics without the linearpower supply).

According to an embodiment, an apparatus includes a circuit forperforming radio frequency identification (RFID). The circuit includesan antenna, RFID electronics including a protective circuit, and anetwork coupled between the antenna and the RFID electronics thatreduces electrical energy provided by the antenna to the RFIDelectronics.

The following detailed description and accompanying drawings provide abetter understanding of the nature and advantages of the presentinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the magnetic flux between a static RFID system (one withouta SLEEP mode) and a dynamic RFID system (one with a SLEEP mode)according to an embodiment of the present invention.

FIG. 2 illustrates a graph of the projected performance enhancement of adynamic RFID system according to an embodiment of the present invention.

FIG. 3 is a flowchart showing a process for reading tags during a readoperation according to an embodiment of the present invention.

FIG. 4A is a top view (cut away), FIG. 4B is a bottom view (cut away),and FIG. 4C is a block diagram, of a token according to an embodiment ofthe present invention.

FIG. 5 is a block diagram of an RFID tag according to an embodiment ofthe present invention.

FIG. 6 is a block diagram of an RFID tag according to an embodiment ofthe present invention.

FIG. 7 is a block diagram of an RFID tag according to an embodiment ofthe present invention using a Varactor diode.

FIG. 8 is a graph comparing output voltage between an existing circuitand the circuit of FIG. 7 according to an embodiment of the presentinvention.

FIG. 9 is a block diagram of an RFID tag according to an embodiment ofthe present invention using Shottky Barrier diodes.

FIG. 10 is a block diagram of an RFID tag 1000 according to anembodiment of the present invention using varactor diodes.

FIG. 11A is a top view (cut away) and FIG. 11B is a bottom view (cutaway) of a token according to an embodiment of the present invention.

DETAILED DESCRIPTION

Described herein are techniques for minimizing energy losses in passiveRFID (radio frequency identification) tags. In the followingdescription, for purposes of explanation, numerous examples and specificdetails are set forth in order to provide a thorough understanding ofthe present invention. It will be evident, however, to one skilled inthe art that the present invention as defined by the claims may includesome or all of the features in these examples alone or in combinationwith other features described below, and may further includemodifications and equivalents of the features and concepts describedherein.

The present application may use one or more of the terms “and”, “or”,and “and/or”. These terms are to be considered to have the same meaningand are to be read as an inclusive or; for example, “X and Y” means thesame as “X or Y” and includes “only X”, and “X or Y” includes “both Xand Y”. When all items joined with “and” are required, or when only oneitem joined with “or” is required, this will be specifically noted(e.g., “both X and Y are required”).

As described above, various range extension solutions may be implementedto increase the read range for de-tuned tags. According to oneembodiment, the specific range extension solution is to include in theRFID tag a magnetically permeable material as described in U.S. patentapplication Ser. No. 12/351,774 titled “Enhancing the Efficiency ofEnergy Transfer to/from Passive ID Circuits Using Ferrite Cores” filedJan. 9, 2009, which is incorporated herein by reference.

This disclosure is organized as follows. First, various embodiments aredescribed that can be applied to “new” RFID tag designs (where thecircuit designer has the freedom to optimize the design of the RFID tagpower supply). Second, various embodiments are described that include acircuit network that can be applied to “existing” RFID tag designs(which have a built-in shunt regulator).

The “New Design” Embodiments

The “new” embodiments use two different approaches to limit the powerconsumed by an individual RFID tag in order to extend the read range:(1) a linear constant-current power supply; and (2) a SLEEP mode.

As noted earlier, existing RFID tags use a shunt regulator to protectthe tag circuitry. In most applications, this circuit is simple andefficacious. In the case where circuits are closely spaced and closelycoupled (e.g., a stack of gaming tokens with ferrite cores such asdescribed in U.S. patent application Ser. No. 12/351,774), however,there may be an unwanted consequence of this architecture: Tags near theexcitation source burn up excess energy as heat—making that energyunavailable for other tokens.

A linear constant current power supply, on the other hand, extracts onlythe current required to operate the tag, thus consuming less of theenergy. This circuitry adds complexity to the RFID tag but theimprovement in read range is marked.

In addition, the energy extracted by a tag can be further reduced usinga SLEEP mode.

FIG. 1 shows the magnetic flux between a static RFID system 101 (onewithout a SLEEP mode) and a dynamic RFID system 102 (one with a SLEEPmode) according to an embodiment of the present invention. The staticRFID system 101 includes an excitation antenna 105 and a stack of gamingtokens 110. The excitation antenna 105 may be part of an RFID reader(not shown). The RFID reader may transmit an excitation signal and, inaccordance therewith, establish an excitation field 103. Each token ofthe stack of gaming tokens 110 may include an RFID tag coupled to theexcitation field and this may create an RFID tag load corresponding toall the tags 107. The RFID tag load absorbs the power from theexcitation field 103 and reduces the excitation field strength. Theexcitation field strength is denoted by the number of arrows used toshow the excitation field 103. The excitation field strength may beinfluenced by the RFID tag load.

The dynamic RFID system 102 includes an excitation antenna 106 and astack of gaming tokens 111. The excitation antenna 106 may be part of anRFID reader (not shown). The RFID reader may transmit an excitationsignal and, in accordance therewith, establish an excitation field 104.Each token of the stack of gaming tokens 111 may include an RFID tag.Initially each RFID tag associated with the stack of gaming tokens 111may be coupled to the excitation field 104. As an RFID tag is read, thereader sends a command signal to the RFID tag to “decouple” from theexcitation field 104. This may be accomplished by any of a number ofmethods—as long as they achieve the intended function of reducing theenergy extracted by the RFID tag from the excitation field 104. Thesemethods include, for example, (1) opening an antenna loop of the RFIDtag; or (2) reducing the power of the RFID circuit (e.g. by putting theprocessor in SLEEP mode). After some time, a portion 109 of the RFIDtags are “off” and the remaining portion 108 of the RFID tags staycoupled to the excitation field 104. The result: while the total outputpower of the excitation field remains unchanged, the flux densitythrough the center of the stack of tokens increases each time a token inthe stack is decoupled. This increased field strength is denoted by thenumber of arrows used to show the excitation field 104. The excitationfield strength may be influenced by the RFID tag load. Since the RFIDtag load of the system 102 is less than the RFID tag load of the staticRFID system 101, the flux density of the dynamic system 102 is greaterthan the flux density of the static RFID system 101. This may increasethe read range of the RFID reader. This increased read range may allowfor the RFID reader to read the portions of RFID tags 111 that werepreviously beyond the static read range. This may allow for a greaterstack of game tokens (or other stacked item containing an embeddeddynamic RFID tag) to be read by an RFID reader for a given excitationpower output.

After the RFID tags are all read, the excitation signal and thereforethe excitation field may turn off. This may allow the RFID tags to resetto a condition in which the RFID tags may become coupled to theexcitation field once more when the excitation field is turned on again.A reset may be induced by a cycling of the excitation field. The timeassociated with the turning off and turning back on of the excitationfield may be dictated by the implementation of the RFID tags and thedynamic circuit which controls how the RFID decouples from theexcitation field.

FIG. 2 illustrates a graph 200 of the projected performance enhancementof a dynamic RFID system according to an embodiment of the presentinvention. The graph 200 shows the power available at a given tagcorresponding to a fixed excitation antenna output power. Line 202representing the power available at this tag for a static system similarto the static RFID system 101 of FIG. 1. The line 202 shows that theexcitation field strength 203 remains constant. The graph 200 shows line201 representing the power available at this same tag for a dynamicsystem similar to the dynamic RFID system 102 of FIG. 1. The line 201shows that as more RFID tags are decoupled from the excitation field (%switch off 204), the more energy is available for additional RFID tags.The increase in flux density through the center of the stack mayincrease the energy available to power tokens higher up in the stack.This may allow for a greater stack of game tokens (or other stacked itemcontaining an embedded dynamic RFID tag) to be read by an RFID readerfor a given excitation power output.

Note that the flux density increases as each tag goes into low-powermode. If we take 20 chips as an example, the reader may correctly read10 tags as soon as the excitation field is energized. Once these 10 tagsare turned “off”, the flux density increases. Let's assume that the nextcommand exchange correctly identifies 5 more tags. The flux density willincrease again as these are also turned “off”. Now let's assume the nextcommand exchange correctly identifies 3 more tags. Again the fluxdensity changes as these tags are turned off. Each time, the energyavailable at a particular tag (as shown on FIG. 2) increases. Hence theterm “dynamic” power absorption.

In the “sleep” state, the tags may present a different load than whenthey are active. According to one embodiment, an active tag presents aload of 10 KOhm, and a sleeping tag presents a load of 500 KOhm.

FIG. 3 is a flowchart showing a process 300 for reading tags during aread operation according to an embodiment of the present invention. Ingeneral, the read operation includes excitation, one or more readcycles, and de-excitation.

In step 301, the reader powers up the excitation signal to energize theRFID tags in the field and then sends a READ command. Numerous protocolsexist for implementing a READ command. The read command may includesynchronization information, as well as information used by each tag tocoordinate its response. Irrespective of the technical details of thecommand protocol used, the goal is to accurately identify each tag inthe field in a timely manner. Each protocol has distinct advantages anddisadvantages that encompass error rates, data throughput, andalgorithmic complexity as well as other tactical details. According toan embodiment, a modified slotted Aloha protocol may be used. Thedetails of this protocol are not relevant to the present invention.

In step 302, all the tags energized by the excitation field thatcorrectly decode the READ command will generate a response. These tagsthen modify the excitation signal to include their responses.

In step 303, the reader attempts to decode these responses from themodified excitation signal. “Collisions” (e.g. multiple tag responsesresponding simultaneously) can prevent the reader from correctlydecoding the tag responses. The command protocol may include errordetection and/or error correction algorithms—but this level of detail is(as noted earlier) not relevant to describe the details of embodimentsof the present invention.

Once the reader has correctly read a specific tag, there is no newinformation that can be extracted from this tag, and the energy used topower its circuitry is wasted. In step 304, the reader sends a SLEEPcommand to each tag that has been read. In step 305, the targeted tagdecodes the SLEEP command and stops transmitting its identity. Up tothis point, the READ and SLEEP commands may be no different than thoseused in other RFID protocols. One feature of an embodiment of thepresent invention is that the SLEEP command is then used to decouple theRFID tag from the excitation field generated by the excitation signal.The result: The energy consumption of each tag that has been read isreduced (e.g., minimized)—strengthening the field and extending the readrange.

Step 306 repeats the data exchange and command sequence (steps 301-305)until all tags that have been energized are read. In step 307, theexcitation field is de-energized, and the read cycle is complete.Re-energizing the excitation field allows the read cycle to repeat asneeded.

One or more of the features described above (linear power regulator anddecoupling SLEEP mode) may be implemented in accordance with otherdesign criteria for embodiments of the present invention. Theseembodiments may include: (1) using discrete components on a rigid orflexible printed circuit, or (2) integrating the required functions intothe design of a custom ASIC. FIGS. 4A-4B show an embodiment of thepresent invention with the functional elements called out.

FIG. 4A is a top view (cut away), FIG. 4B is a bottom view (cut away),and FIG. 4C is a block diagram, of a token 400 according to anembodiment of the present invention. The token 400 may implement the“new design” embodiments discussed above. The token 400 generallyincludes an antenna and tag electronics. More specifically, the token400 includes a printed circuit board 402, an antenna 404, a ferrite core406, a plastic housing 410, a bridge rectifier 412, a current source414, a shunt voltage regulator 416, a receive filter 418, amicroprocessor 420, and a transmit transistor 422.

The token 400 may be a gaming token such as is suitable for use incasinos. The token 400 may be circular with a diameter of 1.55 inches(39.4 mm) and a thickness of 0.125 inches (3.18 mm). These parametersmay be varied as desired.

The printed circuit board 402 may be generally circular in shape, inorder to conform to the form factor of circular gaming tokens. Theprinted circuit board 402 may be of FR-4 material and 0.020 inches inthickness. Alternatively, the circuit may be printed on a flexiblesubstrate such as mylar.

The antenna 404 may be an 8-turn antenna etched on one side (e.g., thebottom) of the circuit board 402. These antennas may be constructed with8 mil traces and 7 mil spacing. The inductance of the 8 turn antenna maybe 3 uH. Antennas with different numbers of turns may be implementedwith a different balance between inductance and resistance, according todesign needs.

Many existing RFID tags use a diode rectifier followed by a voltageclamp to limit the required operating voltage range of the tag andthereby protect the tag from over-voltage. In this embodiment—tailoredfor “new” RFID tag designs—the power supply may be a linear power supplywhere there is a bridge rectifier 412 followed by a current source 414and then a voltage clamp (shunt voltage regulator) 416. Thisarchitecture does not clamp the voltage across the coil as is typicallydone in RFID tags. This linear supply allows tags to operate over abroad range of magnetic field intensities. This allows tags to be readon the top of the stack—where the field is lowest—and near the bottom ofthe stack—where the field is highest. The net effect is an increasedread range. A second set of embodiments—tailored to extracting similarread range benefits using “existing” RFID tag designs—are describedlater in this document.

As noted earlier, the RFID circuitry described in a block diagram formatin FIGS. 4A-4C may be constructed using discrete off the shelfcomponents on a rigid or flexible printed circuit, or integrated intothe design of a custom ASIC. For example, an embodiment using an off theshelf microprocessor 420 may use a PIC microcontroller (e.g.,PIC10F206T-I/OT) from Microchip Technology Inc., Chandler, Ariz. toimplement the functionality described above. The microprocessor 420stores the ID of the token 400, decodes commands from the reader, andencodes the ID of the token 400 onto the 13.56 MHz carrier. Themicroprocessor 420 is coupled to the receive filter 418.

The ferrite core 406 may be as described in U.S. patent application Ser.No. 12/351,774 titled “Enhancing the Efficiency of Energy Transferto/from Passive ID Circuits Using Ferrite Cores” filed Jan. 9, 2009. Theferrite core 406 harnesses the H-field and steers it through the antenna404. The primary physical attribute of the ferrite is its permeability.Any equivalent material that is permeable and effectively steers theH-field through the antenna loop will improve performance. Ferritedevices are commonly used with frequencies up to 1 GHz. According to anembodiment, the ferrite core 406 has a permeability of 125, plus orminus 20% (e.g., between 100 and 150). The ferrite core 406 may also bereferred to as a magnetically permeable material.

The top and bottom of the token 400 may be covered with labels orstickers (not shown) to denote the denomination or other desirableinformation. The label may have a thickness of approximately 0.003inches. This thickness minimizes any air gaps when the tokens arestacked (that is, it helps the ferrite cores in a stack of tokens tofunction as if they were a single monolithic rod of ferrite). Thethickness of the label may be varied, as desired, with correspondingeffects on the read performance. Alternately, the ferrite core may beexposed by using an annular sticker to further minimize any air gaps inthe stack.

The decoupling embodiments discussed above may be implemented with acustom application specific integrated circuit (ASIC) that implementsthe functionality described above. However, the cost and time to developsuch a custom ASIC may be undesirable when implementing otherembodiments. In such embodiments, it may be desirable to useoff-the-shelf RFID chips (for example, the SRF 66V10 from InfineonTechnologies AG) that include a shunt regulator or other protectivecircuit. In such embodiments, it may be desirable that the shuntregulator operate minimally (if at all). The remainder of thisdisclosure describes embodiments that may be used when the RFID chipincludes a shunt regulator.

The “Existing” Shunt Regulator Embodiments

As discussed above, an embodiment of the present invention modifies thebehavior of existing “off-the-shelf” RFID tags in a manner thatovercomes the self-limiting nature of their built-in shunt regulators.Specifically, a network is placed in front of the shuntregulator—between the loop antenna and the RFID chip—to limit theelectrical energy (e.g., current) that reaches the chip.

FIG. 5 is a block diagram of an RFID tag 500 according to an embodimentof the present invention. The RFID tag 500 includes an antenna 502, anRFID chip 504, and a network 506. The RFID tag 500 may otherwise be inthe form factor of a token (e.g., the token 400 of FIG. 4) and mayinclude other components that perform other RFID functions (e.g., asdiscussed above and not repeated for brevity). The antenna 502 maycorrespond to the antenna 404 (see FIG. 4).

The RFID chip 504 may be an off the shelf RFID chip. As such, the RFIDchip 504 includes a protective circuit 510 that protects the RFID chip504 from excessive voltage from an excitation source (e.g., an RFIDreader). The protective circuit 510 may include a shunt regulator. TheRFID chip 504 may be an SRF 66V10 ST microprocessor from InfineonTechnologies AG. The RFID chip 504 may correspond to the microprocessor1120 (see FIG. 11A).

The network 506 may be omitted from RFID applications where the shuntregulator is sufficient protection and where tags are not closely spacedor closely coupled. However, when tags are closely spaced and closelycoupled (as may be the case for a stack of gaming tokens—especiallygaming tokens using ferrite cores to steer the excitation field), thisshunt regulator is detrimental and the network 506 may operated toachieve practical performance goals. Specifically, the network 506limits the current provided to the RFID chip 504 to an amount that issufficient to operate the tag circuitry. This effectively negates therole of the built-in protective circuit 510, minimizing the amount ofexcitation energy that is converted to heat by the protective circuit510 and allowing this energy to remain in the excitation field to powerother RFID tags. The network 506 helps to reduce the amount ofexcitation energy that is absorbed by the RFID chip 504.

Two basic network architectures may be used for the network 506. Onearchitecture is a linear passive network with a constant reactance (e.g.a resistor or a capacitor). Another architecture is a non-linear network(e.g. varactor diodes or Schottky barrier diodes). The specificimplementation details of the network 506 may take several forms and maybe driven by a host of systems considerations such as maximizing systemperformance, minimizing the cost of the tag, and minimizing the designchanges to the reader. Various details for the network 506 are providedbelow.

FIG. 6 is a block diagram of an RFID tag 600 according to an embodimentof the present invention. The RFID tag 600 includes an antenna 502, anRFID chip 504, and a capacitor 606. The RFID tag 600 may otherwisecorrespond to the RFID tag 500 (see FIG. 5) and the details are notrepeated.

The capacitor 606 implements the network 506. Experimental resultsshowed that different capacitances worked best for different excitationfield strengths. Thus, for a given reader power output, the optimumcapacitor for an RFID tag at the top of a stack of gaming tokens isdifferent than the optimum capacitor for an RFID tag at the bottom of astack of gaming tokens. For example, for a reader antenna output of 1watt, and a stack of 30 tokens, the optimum capacitor value varied from1 pF to 10 pF. Since all tokens in the stack should use identicalcapacitors, it was necessary to select a suitable value. A capacitor 606in the range of 1 pF to 10 pF worked reasonably well. A capacitor 606 inthe range of 3.3 pF to 4.7 pF also worked reasonably well.

One approach to the construction of the network is simply to add adiscrete capacitor as shown in FIG. 6. An alternate embodiment is to usecopper layers in the printed circuit board to achieve the same purpose.This is particularly feasible if the printed circuit board isconstructed from flexible film as is common on RFID inlays. The inlaymay be made from a biaxially-oriented polyethylene terephthalate (boPET)polyester film such as the Mylar™ material. The capacitor 606 may beformed on the inlay by metal deposition. A capacitor 606 formed in thisway thus eliminates the need for a discrete component.

In addition to limiting the power available to the RFID chip 504, thecapacitor 606 has the following tangible benefits. First, it de-couplesadjacent tags—thereby minimizing shifts in resonance that can degradesystem performance. Second, it allows one to increase the output powerof the reader (to further extend read range) without damage to thecircuitry in the RFID chip 504 circuitry.

A second approach to the construction of the network is to attempt tomatch the reactance of the network to the strength of the excitationfield. Using the same example as before, this network would ideally looklike a capacitance of 1 pF at the bottom of a stack of tokens and acapacitance of 10 pF at the top of a stack of 30 tokens. Two circuitarchitectures to achieve this performance are described: (1) a VaractorDiode circuit, and (2) a Shottky Barrier Diode circuit.

FIG. 7 is a block diagram of an RFID tag 700 according to an embodimentof the present invention using a Varactor diode. The RFID tag 700includes an antenna 502, an RFID chip 504, and other components that mayotherwise correspond to the RFID tag 500 (see FIG. 5), the details ofwhich are not repeated.

The RFID tag 700 includes a varactor 706 to implement the network 506(see FIG. 5). One feature of the design is to extract sufficient energyfrom the excitation field to run the RFID circuit 504 but use thecharacteristics of varactors to limit the current drawn by the RFID chip504 and thereby minimize the losses (heat).

In this implementation, L1 represents the loop antenna 502 and D1 is thevaractor 706. The peak voltage on the loop antenna 502 is detected usingdiodes D2 710 and D3 712 which charge capacitor C1 714 through resistorR3 716. For a particular varactor diode 706, there is a an optimal biasfor a given voltage on the loop antenna 502. The purpose of D3 712 is toshift the bias closer to the optimum; additional diodes may be used toachieve this shift according to various specific embodiments. ResistorsR3 716 and R2 718 form a voltage divider for more control of the bias.The resistor R2 718 also acts as a bleed resistor to track changes inthe loop antenna 502 voltage. Resistor R1 720 is a high value resistorthat applies the bias voltage held on capacitor C1 714 to the cathode ofthe varactor 706. The capacitor 722 provides AC coupling to the RFIDchip 504. This allows the voltage on the cathode of the varactor diode706 to increase without being affected by the RFID chip 504.

FIG. 8 is a graph comparing output voltage between an existing circuitand the circuit of FIG. 7 according to an embodiment of the presentinvention. The figure shows the theoretical output voltage of the loopantenna (e.g., 502) as a function of the strength of the excitationfield generated by the reader and received by the tag. The line 802shows that with no varactor, the output voltage increases in proportionto the ambient field strength—requiring a shunt regulator to protect thetag circuitry. The line 804 shows that with the varactor circuit (e.g.,706 and related components in FIG. 7), the output voltage rises quickly(using the inherent capacitance of the varactor 706 to tune the LCcircuit of the loop antenna 502) and then levels off at a much smallerslope. The circuit is designed such that the bias described above isshifted such that the output voltage is above Vth—the threshold requiredto energize the RFID chip. The result: the circuits in the RFID chip 504are protected when the excitation field is strong but the excess energyavailable is not consumed by the shunt regulator in the RFID chip 504.

FIG. 9 is a block diagram of an RFID tag 900 according to an embodimentof the present invention using Shottky Barrier diodes. The RFID tag 900includes an antenna 502, an RFID chip 504, and other components that mayotherwise correspond to the RFID tag 500 (see FIG. 5), the details ofwhich are not repeated.

The RFID tag 900 includes two Schottky barrier diodes 902 and 904 inseries with the RFID chip 504. The capacitance of these diodes 904 and904 changes as reverse bias is applied to them. Thus, when the voltageon the top of the antenna 502 is (for example) +1 volt, the first diode902 is forward biased, which simply reduces the voltage to about 0.7volts at the cathode of the second diode 904. Using the specificationsfor a BAT54 diode (Fairchild Semiconductor Corp., South Portland, Me.),this condition results in a capacitance of about 9 pF. If, on the otherhand, the voltage on the top of the antenna 502 is 10 volts, the reversebias on the second diode 904 is 9.7 volts and the resulting capacitanceis 4 pF. If the voltage at the top of the antenna 502 is negative, theroles of the two diodes 902 and 904 is reversed with the same netresult. The resistor R1 906 ensures that a reverse bias can be generatedand may be omitted depending on the internal design of the RFID chip504.

Multiple variants on the architecture of FIG. 9 are possible. Forexample, two varactor diodes may be in parallel with the Schottkybarrier diodes 902 and 904, with the cathodes of all four diodes tiedtogether. The Schottky diodes 902 and 904 in this case may have lowcapacitance so that the variable capacitance is determined by thevaractors. In this case, the Schottky barrier diodes 902 and 904 preventthe varactor diodes from conducting current in the forward direction.Similarly, one could use transistors in the network because theircollector capacitance also varies with voltage.

FIG. 10 is a block diagram of an RFID tag 1000 according to anembodiment of the present invention using varactor diodes. FIG. 10 issimilar to FIG. 9, with the Schottky diodes (902 and 904 in FIG. 9)replaced with varactor diodes 1002 and 1004, and a resistor 1006 addedto connect their cathodes to the bottom of the coil 502. Similarly,varactor diodes may be combined with Schottky diodes to get the desiredresponse.

FIG. 11A is a top view (cut away) and FIG. 11B is a bottom view (cutaway) of a token 1100 according to an embodiment of the presentinvention. The token 1100 may implement the “existing design”embodiments discussed above. The token 1100 is similar to the token 400(FIGS. 4A-4C), and for conciseness the similar elements will not bedescribed again. The token 1100 may otherwise include the elements ofFIG. 5 (the antenna 502, the RFID chip 504, and the network 506).

The above description illustrates various embodiments of the presentinvention along with examples of how aspects of the present inventionmay be implemented. The above examples and embodiments should not bedeemed to be the only embodiments, and are presented to illustrate theflexibility and advantages of the present invention as defined by thefollowing claims. Based on the above disclosure and the followingclaims, other arrangements, embodiments, implementations and equivalentswill be evident to those skilled in the art and may be employed withoutdeparting from the spirit and scope of the invention as defined by theclaims.

What is claimed is:
 1. A method of performing radio frequency identification (RFID), comprising the steps of: (a) generating, by an RFID reader, an excitation signal that includes a first read command; (b) generating, by a first plurality of RFID tags, a first plurality of responses as part of the excitation signal; (c) decoding, by the RFID reader, the first plurality of responses from the excitation signal; (d) transmitting, by the RFID reader, a sleep command to each of the first plurality of RFID tags using the excitation signal; (e) decoupling, by the first plurality of RFID tags, from an excitation field generated by the excitation signal, in response to the sleep command; and (f) repeating (a), (b), (c), (d) and (e) for a second plurality of RFID tags.
 2. An apparatus including a circuit for performing radio frequency identification (RFID), comprising: an antenna; and RFID electronics, coupled to the antenna, that is configured to activate in response to receiving an excitation signal, to generate a response as part of the excitation signal, to receive a sleep command as part of the excitation signal, and to decouple from an excitation field generated by the excitation signal, in response to the sleep command.
 3. An apparatus including a circuit for performing radio frequency identification (RFID), comprising: a plurality of RFID tags including a first set and a second set, wherein each of the plurality of RFID tags includes: an antenna, and RFID electronics coupled to the antenna, wherein the first set of the plurality of RFID tags is configured to activate in response to receiving an excitation signal, to generate a first plurality of responses as part of the excitation signal, to receive a first plurality of sleep commands as part of the excitation signal, and, in response to the first plurality of sleep commands, to decouple from an excitation field generated by the excitation signal, wherein the second set of the plurality of RFID tags does not receive the excitation signal until the first set of the plurality of RFID tags has been decoupled from the excitation field, and wherein the second set of the plurality of RFID tags is configured to activate in response to receiving the excitation signal, to generate a second plurality of responses as part of the excitation signal, to receive a second plurality of sleep commands as part of the excitation signal, and to decouple from an excitation field generated by the excitation signal, in response to the second plurality of sleep commands.
 4. An apparatus for performing radio frequency identification (RFID), comprising: an RFID reader that includes a reader antenna, wherein the RFID reader is configured to generate an excitation signal; and a plurality of RFID tags including a first set and a second set, wherein each of the plurality of RFID tags includes: a tag antenna, and RFID electronics coupled to the tag antenna, wherein the first set of the plurality of RFID tags is configured to activate in response to receiving the excitation signal, and to generate a first plurality of responses as part of the excitation signal, wherein the RFID reader is configured to decode the first plurality of responses from the excitation signal, and to transmit a first plurality of sleep commands to the first set of the plurality of RFID tags using the excitation signal, wherein the first set of the plurality of RFID tags is configured to receive the first plurality of sleep commands as part of the excitation signal, and, in response to the first plurality of sleep commands, to decouple from an excitation field generated by the excitation signal, wherein the second set of the plurality of RFID tags does not receive the excitation signal until the first set of the plurality of RFID tags has been decoupled from the excitation field, wherein the second set of the plurality of RFID tags is configured to activate in response to receiving the excitation signal, to generate a second plurality of responses as part of the excitation signal, wherein the RFID reader is configured to decode the second plurality of responses from the excitation signal, and to transmit a second plurality of sleep commands to the second set of the plurality of RFID tags using the excitation signal, and wherein the second set of the plurality of RFID tags is configured to receive the second plurality of sleep commands as part of the excitation signal, and, in response to the second plurality of sleep commands, to decouple from the excitation field generated by the excitation signal. 